This invention relates to fluid flow through pumps. More specifically, this invention relates to determining fluid cavitation and an estimate of mechanical seal failure caused by such cavitation.
Fluid pumps and their associated technology are well-known in the art. Pumps typically are incorporated into fluid transport systems to change the direction of the fluid flow or to increase rate or pressure of the fluid flow. Ideally, fluid transport systems require little or no maintenance. One feature of fluid pumps is that the fluid being pumped is used as a lubricant to reduce the wear on the pump""s internal components. For example, the pumped fluid provides a liquid surface boundary layer, which prevents the components of mechanical seals from coming into contact.
When a low pressure condition occurs in a pump, vapor bubbles exit the pumped fluid and begin a process, i.e., cavitation that can cause failure in the pump. In one case, vapor bubbles impact with, and implode on, the impeller blades of the pump. Because of the high speed of the impeller blades, the continuous impact of vapor bubbles can damage the impeller blades. Furthermore, the vapor bubbles have an insufficient consistency to maintain a boundary layer between mechanical seal components. Thus, the mechanical seal components can come into contact, which generates heat and wear.
Methods of determining cavitation are well known in the art. One method, for example, measures the pump""s suction pressure and pump temperature. From these measurements and known vapor pressure/temperature curves, a Net Positive Suction Head Available (NPSHa) is computed. The NPSHa is then compared to an NPSHr (Net Positive Suction Head Required) for the measured pump speed. When NPSHr is greater than NPSHa, the fluid in the pump is deemed to be cavitating. A second method identifies high frequency noise, which is indicative of cavitation, in a pump bearing housing, a suction flange case or a mechanical seal chamber. A third method is to measure pressure and temperature in the mechanical seal chamber and infer vaporization across the mechanical seal face. Each of these methods had known disadvantages. The first method requires measurements of at least four variables, which imposes additional hardware costs on the pump. The second method can falsely indicate cavitation as other conditions can create high frequency noises. The third method provides an indication of vaporization across the mechanical seal face and not pump fluid cavitation.
Hence, there is a need to provide a simple and reliable method of determining pump cavitation and when possible an estimate of the degradation in seal life caused by cavitation and the remaining useable life of the seal.
A method and system for determining cavitation in a pump having a known non-cavitating dynamic pressure measure, is disclosed. In accordance with the principles of the invention, fluctuations of the pressure with the pump, i.e., the dynamic pressure, are recorded and compared to a known cavitation alarm dynamic pressure. The cavitation alarm dynamic pressure is a known percentage of the non-cavitating pressure measurement. When measured dynamic pressure is determined to be less than the cavitation alarm pressure, an indicator is made available, i.e., output, to indicate the occurrence of cavitation. In a further aspect of the invention, remaining seal life can be determined by maintaining the time cavitation is present and determining a seal degradation time relating to the pump cavitation time and a seal degradation factor. The seal degradation time can then be removed from the expected operational seal life to determine the remaining usable seal life.